The Toronto Stock Exchange lost 2.8 per cent of its value Monday, in a rout that saw the S&P TSX Composite index drop 373 points.
The index has wiped out all its gains since October, 2013, the last time it closed at these levels, Bloomberg reports.
The market was dragged down by Montreal-based Valeant Pharmaceuticals, which for some time this summer was Canada's largest company. Democrats in the U.S. Congress are reportedly pushing for an investigation of the company over price hikes earlier this year.
The New York Stock Exchange did somewhat, but little, better. The Dow Jones had its worst drop in nearly a month as investors worried about more signs of weakness in China's economy.
The index dropped 312 points, or 1.9 per cent, to 16,001 Monday.
The Standard & Poor's 500 index sank 49 points, or 2.6 per cent, to 1,881. The Nasdaq composite lost 142 points, or 3 per cent, to 4,543.
Biotechnology stocks, until recently a high-flying sector, continued to drop sharply.
Mining, energy and basic materials stocks also fell hard. Tumbling prices for oil, copper and other commodities have hurt profits at those companies.
The losses were widespread, with 98 per cent of the stocks in the S&P 500 index ending lower.
Bond prices rose. The yield on the 10-year Treasury note fell to 2.10 per cent.
— The Huffington Post Canada, with files from The Associated Press